The aim of this study was to assess patients’ functioning and sleep quality during extended-release quetiapine fumarate (quetiapine XR) maintenance treatment. A double-blind, randomized-withdrawal maintenance study of quetiapine XR monotherapy was carried out in patients with major depressive disorder. Following 4–8 weeks of open-label quetiapine XR and 12–18 weeks of open-label quetiapine XR stabilization (50, 150, or 300 mg/day), eligible patients were randomized to quetiapine XR (50, 150, or 300 mg/day) or placebo. Secondary variables of the Sheehan Disability Scale (SDS) and the Pittsburgh Sleep Quality Index (PSQI) were used to assess functioning and sleep quality and are reported here. Quetiapine XR significantly maintained functioning versus placebo. Changes in the least squares means (LSM) from randomization in the SDS total scores were as follows: −0.45, quetiapine XR (P<0.05), versus 0.44, placebo. Quetiapine XR significantly maintained SDS domains ‘social life/leisure’ (−0.19; P<0.05) and ‘family life/home responsibilities’ (−0.22; P<0.05) versus placebo (0.13 and 0.10, respectively). Quetiapine XR significantly maintained sleep quality (LSM change in PSQI total scores: 0.06, quetiapine XR vs. 1.35, placebo; P<0.001), with five of seven PSQI components being significant for quetiapine XR versus placebo. In conclusion, quetiapine XR (50–300 mg/day) monotherapy better maintains overall functioning and sleep quality than placebo in patients with major depressive disorder.